Spinal discs provide support between adjacent vertebrae in a spinal column. Over time, discs can rupture, degenerate or protrude outside of their normal space as a result of injury, degradation or disease. In such cases, the condition of the disc can be weakened or compromised to the point that the intervertebral space around the disc collapses. Changes in disc shape can cause the spine to lose its normal curvature, create impingement of nerves in the disc space, and result in chronic back pain.
A number of surgical procedures can be performed to treat damaged discs. In one procedure, the degenerative disc is removed, and the remaining adjacent vertebrae are connected by fusion. This procedure may involve the use of an intervertebral body spacer or cage in conjunction with bone graft material. The spacer is inserted between the vertebrae to create and maintain a desired spacing between the vertebrae. The bone graft material promotes fusion of the vertebrae for long term stability. During the fusion process, it is desirable to inhibit relative movement of the spacer and adjacent vertebrae. To this end, many spacers are provided with anchor members, such as bone screws, that are inserted through the spacer and into the vertebrae to stabilize the spacer between the vertebrae.
Anchor members that secure spacers between vertebrae can loosen over time in response to micro motion and other factors. Once anchor members are loosened, they may back out of the vertebrae and no longer hold the spacer in a stable condition. In addition, loosened anchor members can project from the vertebral space and contact tissue, blood vessels or organs, causing damage.
There have been a number of attempts to reduce the occurrence of screw backout. For example, U.S. Pub. No. 2008/0249569 describes implants with face plates that are attachable over the implants to inhibit anchors from backing out of the implants. In one embodiment, the implant has anchor apertures that pass through the front surface of the implant. The side surfaces of the implant include recesses with attachment features that connect with attachment features on the face plate. The face plate is attachable over the front surface of the implant to cover the anchor apertures. In this position, the faceplate inhibits the anchor members from backing out of the implant. The faceplate is attached over the implant once the implant is inserted into the vertebral space, and after the anchor members are driven into the vertebrae.
U.S. Pub. No. 2006/0085071 discloses another spacer with bone screws that have male threads on the screw heads. The screws are inserted through a front plate that has bore holes. The bore holes have short threaded sections at the entrances of the bore holes which mate with the screw heads. According to the inventors, the threaded engagement anchors the screws in the front plate in a rigid manner. After the screws are anchored in the front plate, a separate securing plate is attached over the bore holes and screws to inhibit the screws from backing out of the spacer.
U.S. Pub. No. 2008/0249575 discloses interbody spacers with deformable locking rings to inhibit bone screws from backing out of the screw passages. In general, the locking rings deflect radially to allow the bone screws to advance into the passage to a certain point. Once the bone screws pass through the locking rings, the locking rings return to a relaxed configuration that traps the bone screws in the passages and inhibits the screws from backing out. The screw passages are relatively short and appear to engage only a small portion of the screw heads.
U.S. Pub. No. 2009/0030520 discloses an interbody spacer with bone screws having a special thread configuration. Each bone screw has a head and a shaft with two sections. The shaft includes an external thread at one section and a clearance groove at another section that separates the external thread from the head. The external thread is designed to engage an internal thread inside the screw passage in the spacer, and then disengage from the internal thread as the screw is advanced into the bone. Once the screw is fully seated in the spacer, the external thread portion disengages from the internal thread in the spacer, and the clearance groove is aligned with the internal thread. The bone screw is then rotated another 90°-270° in the seated position to separate the orientation of the external thread from the internal thread runout. Because the external thread is rotationally separated and disengaged from the internal thread, the external thread abuts an end face on the spacer, preventing the screw from backing out through the passage.
One drawback to known designs is that they provide no mechanism to assist removal of the bone screw from the spacer. A bone screw may need to be removed for a number of reasons. For example, the bone screw and spacer may need to be removed where the patient's condition changes or where the surgeon decides that the bone screw needs to be replaced with a shorter or longer screw. Removing the bone screw is almost impossible if there is no mechanism to get the screw out of the bone and out of the spacer. This is particularly true with designs that trap the head of the screw behind a locking ring or other obstruction inside the spacer, as shown for example in U.S. Pub. No. 2008/0249575. Locking rings are typically designed to permanently trap the screw head in the spacer behind the locking ring. Even in designs where the screw head is unobstructed, the typical design still provides no mechanism to help remove the screw out of the passage. In U.S. Pub. No. 2009/0030520, for example, the bone screw is driven into bone and rotated until its external thread misaligns with the internal thread in the spacer. The external thread abuts the end face of the spacer and cannot be reengaged with the internal thread to remove the screw. Moreover, the screw head is countersunk in the screw passage, providing no way to grab onto the head and apply axial force on the head to remove the screw.
Another drawback to many known designs is that the spacer provides no control against overtightening of bone screws. Typically, a screw hole is tapped and/or pre-drilled into the bone. As a bone screw is driven into the hole in the bone, a delicate thread is formed in the interior of the hole that mates with the thread on the screw. The delicate thread in the bone will be damaged and stripped if the screw is overtightened, When the thread in the bone is stripped, the thread can no longer assist in removing the screw in the event the screw is unscrewed from the bone. Many known spacers lack a positive stop mechanism to limit rotation of the screw and prevent overtightening and stripping of the bone thread. In U.S. Pub. No. 2009/0030520, for example, the screw is intended to be rotated through some unspecified angle after it is seated in the spacer to create a compression lock. The screw is intended to be rotated after it is seated to misalign the external thread on the shaft and internal thread in the passage. Spacers of this kind do not have a positive stop and therefore allow the screw to be rotated indefinitely. This is problematic because a surgeon may unknowingly overtighten the screw, or purposely overtighten the screw rather than face the risk of not tightening the screw enough. In either event, the bone thread will be damaged, compromising the integrity of the screw hole.
Another drawback in many designs is the need for separate components to center the bone screws in the passages and control the trajectory of each screw as it is driven through the spacer. Centering each screw in its respective bore hole or passage is important, particularly in designs having locking threads on the screw heads. External threads on screw heads must align and mate with the internal threads in the bore holes or passages when the screw heads enter the passages. This mating does not occur until the screw is almost completely inserted into the passage (i.e. when the head enters the passage). Because the passages are wider than the screw shanks, and because the screw shanks are not engaged with any part of the spacer, there is play that allows the screw to change alignment as it advances through the passage. To maintain proper screw alignment and trajectory, many designs, including some mentioned above, utilize a separate drill guide, guide pin or other implement to ensure that the screws remain centered in each hole. This adds additional steps to the procedure, and burdens the surgeon with additional components that must be handled during surgery.
Another drawback in many designs is that the spacer provides little support and stability to the screws. In U.S. Pub. No. 2008/0249575 and U.S. Pub. No. 2009/0030520, for example, only a small section of the screw head engages the inside passage of the spacer. The screw head represents a relatively small portion of the screw's length. Most of the screw extends through the passage and does not engage the spacer at all, allowing the screw shank to move laterally or polyaxially in the screw passage under load. As a result, the spacer contributes very little stability to the screw as it undergoes flexion/extension, medial/lateral bending and axial rotation.
Still another drawback in many designs is their need for a separate component, such as a cover plate or cover screw, placed over the anchors to inhibit the anchors from backing out of the implant. Implants that utilize detachable covers, like U.S. Pub. No. 2008/0249569, require the surgeon to handle additional instrumentation and components, and perform additional steps that can prolong surgery.